Not applicable.
Not applicable.
Pneumotachometers, or flow sensors, are used to measure inhalation and exhalation flow rates. A spirometer is a medical device which uses flow rate signals from a pneumotachometer to measure the volume of air entering and leaving the lungs and conduct specific pulmonary function assessments.
One type of pneumotachometer is a differential pressure pneumotachometer in which an air-resistive element located in the air flow path creates a pressure drop which is proportional to the air flow rate. A pressure transducer converts the differential pressure across the resistive element into an electrical signal indicative of air flow rate. The flow rate signal can be integrated to provide an indication of breath or xe2x80x9cflowxe2x80x9d volume.
Respiratory flow rates and breath volumes are measured for use in various medical diagnostic tests. For example, heart rate variability analysis, which is used to evaluate a patient""s autonomic nervous system function, utilizes respiratory flow rate measurement. In heart rate variability analysis, the patient""s autonomic nervous system is exercised by performing various breathing maneuvers, or tests. Two such tests are the Valsalva test and the metronomic deep breathing test. The Valsalva test requires that the patient forcibly exhale at a predetermined pressure, such as 40 mmHg, for a predetermined duration, such as 15 seconds, during which the heart rate is monitored. Thereafter, the patient rests for a predetermined duration. The result of the Valsalva test is a ratio of the highest heart rate (as indicated by the shortest R-R interval in the patient""s ECG signal) during the breathing maneuver to the lowest heart rate (as indicated by the longest R-R interval) during a rest period after the maneuver. In accordance with the metronomic deep breathing test, which is sometimes referred to as the E/I test, the patient is instructed to breathe deeply at a frequency of 6 cycles/minute, which has been shown to produce predictable heart rate variability in healthy individuals. The result of the metronomic deep breathing test is a ratio of the average of the heart rate peaks from the ECG signal to the average of the heart rate troughs. Measurement of the patient""s breath flow rate during these breathing maneuvers is used to monitor compliance with the desired breathing maneuver and thus, to ensure accurate testing.
Various differential pressure pneumotachometers are available. One such device is a disposable pneumotach from Advanced Biosensor of Columbia, S.C. This device has an inlet into which a patient breathes, an outlet through which air exits or enters the device depending on whether the patient is exhaling or inhaling, respectively, and a thin membrane positioned between the inlet and the outlet, so as to divide the device into an inlet chamber and an outlet chamber. The membrane is comprised of a fiber mesh, such as a nylon mesh, which restricts the air flow enough to cause a pressure drop from one side of the membrane to the other. A first port for sensing the inlet chamber pressure is located in the inlet chamber and a second port for sensing the outlet chamber pressure is located in the outlet chamber. Each pressure port is adapted for coupling to an input of a differential pressure transducer through a respective tube. In some testing applications, such as the Valsalva test, a plug is placed on the outlet in order to permit a predetermined air pressure to be achieved by further restricting air flow. The pneumotachometer is disposable, but the tubes coupled between the pneumotachometer and the pressure transducer as well as the Valsalva plug are reusable.
One problem with such a pneumotachometer is possible contamination. As the patient breathes into the pneumotachometer, contagions can enter the reusable tubes through the pressure sensing ports and can also contaminate the reusable Valsalva plug, thereby potentially causing cross-contamination between patients. Further, condensation entering the tubes can deteriorate the performance of the pressure transducer and other processing electronics.
A disposable bacterial filter device, such as the VIRO III disposable filter of A-M System, Inc. of Carlsborg, Wash., is sometimes used in conjunction with a pneumotachometer in order to reduce contamination. The disposable filter device includes a bacterial filter material which acts as a barrier to bacteria and viruses. However, use of a bacterial filter device in conjunction with a pneumotachometer increases the cost and decreases the ease of use of the device.
According to the invention, a pneumotachometer includes an inlet, an outlet, a resistive element positioned between the inlet and outlet to divide the device into an inlet chamber and an outlet chamber, and a pressure port disposed in the outlet chamber through which the static pressure in the inlet chamber is sensed. Since the pressure port for sampling the inlet chamber pressure is located in the outlet chamber and is isolated from the inlet chamber by the resistive element, the resistive element serves to isolate the pressure port, and also the reusable tubing coupled to the pressure port, from contagions. With this arrangement, the likelihood of that contagions introduced into the device through the inlet will contaminate apparatus coupled to the pressure port is reduced.
In one embodiment, a pressure sampling channel has a first portion disposed in the inlet chamber in gaseous communication with the inlet chamber through at least one aperture and a second portion disposed in the outlet chamber in gaseous communication with the first channel portion and with the pressure port. The first and second portions of the pressure sampling channel are divided by a portion of the resistive element. Because there is negligible air flow through the pressure sampling channel, there is negligible pressure gradient across the portion of the resistive element located in this region. As a result, the pressure sampled via the pressure port provides an accurate indication of the static pressure in the inlet chamber.
In one embodiment, the aperture in the inlet portion of the sampling channel is provided in the form of slots. With this arrangement, manufacture of the pneumotachometer by injection molding is facilitated.
According to a further aspect of the invention, a pneumotachometer having an inlet and an outlet and a resistive element positioned between the inlet and outlet to divide the device into an inlet chamber and an outlet chamber is provided with a resistive element in the form of a bacterial filter material. The bacterial filter material comprises a web of electrostatically charged, hydrophobic fibers. Use of a bacterial filter material for the resistive element improves the isolation between the patient""s mouth and a pressure port located in the outlet chamber. The bacterial filter material additionally serves as a barrier between the patient""s mouth and the Valsalva plug covering the outlet during the Valsalva maneuver.
According to a further aspect of the invention, one or more pressure ports of the pneumotachometer is covered by a bacterial filter material. This arrangement further reduces contamination of apparatus coupled to the covered pressure port.
The pressure in the outlet chamber may be sensed through a second pressure port or may be presumed to be at ambient. In one embodiment, the second pressure port is disposed in a wall of the outlet chamber and may or may not be protected by a bacterial filter. In an alternative embodiment, the second pressure port is disposed in a wall of the inlet chamber and is in isolated gaseous communication with the outlet chamber through a pressure sampling channel or tube.